The challenge with modeling

Thermal Barrier Coating optical

properties is to account for the

significant amount of scattering

that occurs due to the intrinsic

inhomogeneities of porous ceramic

materials that are used to lower

thermal conductivities. In addition,

it is necessary to consider

absorption of light as it travels

through the coating. A well-known

model that combines both factors

is the Kubelka-Munk model. The

following equations are describing

the distribution of intensities of

excitation and emission lights as

the laser beam penetrates TBC that

contains luminescent dopants [2]:

The model has been further

extended to predict the

luminescence coming from the

thermally grown oxide (TGO) that

forms when aging the material:

Innovative specimens including

dopant in the bond coat that

diffuses to the thermally grown

oxide are currently being prepared

for temperature measurements at

the top coat - bond coat interface.

The monitoring of temperature at

this key location can help

controlling the degradation

mechanisms occurring in operating

conditions on turbine blades and

allow for more efficient engines.

One of the main work on this

model is the integration of decay

time for numerical predictions of

the collectable 𝐽𝑙𝑢𝑚 0, 𝑡 , in presence

of a gradient of temperature

based on the following equation:

The equation includes the

exponential decay that is

characteristic of luminescence. The

latter equation is providing very

promising results to correlate

temperature readings obtained

with decay time phosphor

thermometry and its accurate

position in the coating, allowing

for reduction of errors in the

evaluation of the temperature

distribution. Based on this results,

a reverse model has been

constructed to predict temperature

at any point in the TBC using a

single point measurement.

The plot presented below show the

results of equivalent position for

the temperature measurement and

collectable intensity using the

model for 325 TBC layer

configurations on YSZ:Dy

The precision of the previous

results is to be related to the

effects of the input gradient of

temperature on the model. The

sensitivity of the model to the

input surface temperature and

internal gradient for the

configuration (C1) for YSZ:Dy is

shown below. It has been found

that for the usable range of

temperature of this material

(800K–1450K) the equivalent

position remains constant (±8K

error) which allows to retrace

temperature and internal gradients

simply based on Phosphor

Thermometry.

Thermal barrier coatings (TBCs)

are used to protect turbine

components from the extremely

hot gas flow, which may be above

the component materials melting

point. Accurate temperature

measurements enable precise

lifetime predictions, which favor

safety and efficiency. In-situ

monitoring of in-service turbine

components is ideal; a promising

method is Phosphor

Thermometry which uses the

luminescence decay of doped

coatings stimulated by a pulsed

laser. There are various

configurations of candidate

phosphors and host materials,

but it is crucial to ensure both

sensing and integrity needs are

met.

BACKGROUND &

MOTIVATION

METHOD :

LUMINESCENCE DECAY

MODELING COLLECTABLE LUMINESCENCE USING

FOUR-FLUX KUBELKA-MUNK MODEL

INTEGRATION OF DECAY TIME

The Phosphor Thermometry

instrumentation at the University

of Central Florida has been

constructed in collaboration with

Dr. Heeg at Lumium, The

Netherlands, and is composed of

a switchable 355 nm / 532 nm

wavelength pulsed laser that

excites doped specimens.

Neutral density and bandpass

filters are used to collect the

luminescence. A photomultiplier

tube is used to convert the

photons into a detectable electric

signal. The data collection is

then processed through MATLAB.

The sensitivity of the

temperature measurement is

proper to each configuration and

based on the variation of decay

time with temperature. The

equipment has been designed so

it is adaptable to the sample

positioning and portable.

REFERENCES &

ACKNOWLEDGMENTS[1] Fouliard, Q. P., Jahan S. A., Rossman L., Warren

P., Ghosh R., Raghavan S., Configurations for

Temperature Sensing of Thermal Barrier Coatings,

International Conference on Phosphor Thermometry,

25-27 July, 2018, Glasgow, UK

[2] Fouliard, Q. S. Haldar, R. Ghosh, S. Raghavan,

Modeling Luminescence Behavior for Phosphor

Thermometry Applied to Doped Thermal Barrier

Coating Configurations, Applied Optics (2019) -

Submitted

[3] Pilgrim, C. C., J. P. Feist, and A. L. Heyes. "On the

effect of temperature gradients and coating

translucence on the accuracy of phosphor

thermometry." Measurement Science and

Technology 24.10 (2013): 105201.

This material is based upon work supported by the

U.S. Department of Energy, National Energy

Technology Laboratory, University Turbine Systems

Research (UTSR) under Award Number: DE-

FE00312282.

Sensing TBC configurations have

been separated into two

categories:

(C1) is easier to manufacture,

does not add any extra interface

and provides higher

luminescence intensities. (C2)

gives higher accuracy of

luminescence signal [1].

Department of Mechanical and Aerospace Engineering, College of Engineering and Computer Science, University of Central Florida,

Orlando, Florida, USA

Quentin Fouliard, Sandip Haldar, Ranajay Ghosh, Seetha Raghavan

OBJECTIVES

DOPED LAYER TBC

CONFIGURATIONS

Data collection through doped

TBCs at high temperature.

Addition of a PMT with a different

bandpass configuration for the

combination of the decay and the

intensity ratio methods.

Specimen fabrication and residual

strain characterization of the doped

TBCs to ensure the mechanical

integrity of the specimens.

Quantify the luminescence

intensity for any TBC configuration.

Predict the location into the TBC

of the Phosphor Thermometry

temperature output.

INSTRUMENTATION

Fan

Low-Pressure

Compressor

High-Pressure

Compressor

Combustor

Low-

Pressure

Turbine

High-

Pressure

Turbine

& Coated

Blade

Thermal

Barrier

Coating

Image

courtesy: G

E

FUTURE WORK

MATERIALS

YSZ:Eu (1% Eu2O

3, 8% Y

2O

3, 91%

ZrO2) TBC coupons were prepared

by Air Plasma Spray at the Florida

Institute of Technology to validate

initial results of Kubelka-Munk

based models. Er, Dy, Sm and Cr

dwdadw

250

μm

Doped top coat

Undoped top coat

Bond coat

Substrate

(C1) (C2)

Thermally grown oxide

are other

dopants that

will be made

for Phosphor

Thermometry.

This instrument is combined with

an infrared heater (model E4 from

Precise Control Systems Inc, MN,

USA) that is capable of heating the

specimens up to 1300˚C to

reproduce TBC real service

conditions. Measurements have

been taken on YSZ:Eu powder and a

fit allows for retracing temperature.

YSZ:Eu top

coated sample